The invention pertains to a shielded jacket for electrical and electronic cables, and more particularly, to a novel construction of a jacket having two separate shields which may be chosen to optimally shield against different frequencies of electromagnetic radiation.
It is common to use a shielding jacket placed over one or more conductors when either it is necessary to contain electromagnetic emissions from the conductors or to protect the conductors from external electromagnetic emissions. In accordance with Gauss' Law, it is desirable to surround the protected conductors with a grounded conductive surface. As placing a solid metal tube around a conductor would often be highly impractical, the typical shielding jacket uses some form of wire mesh electrical shield which surrounds the conductors. In a closable jacket the shield is generally formed in an elongated, approximately rectangular shape and then wrapped around the conductor to form a tube. Some form of flexible casing similarly surrounds the shield. When such a mesh shield is used, the size and type of mesh are chosen based upon the specific ranges of frequency of the emissions which are to be shielded against. Typically, a mesh does a poor job of shielding against emissions of a wave length substantially smaller than the size of the openings in the mesh.
In a prior shielding jacket manufactured by the applicant, the rectangular shield is formed by flattening a knit wire sleeve. In the prior product a flexible polyvinyl chloride (PVC) casing in the form of a long sheet with a zipper along its edges is provided to surround the shield. A heavy braided wire conductor (braid) is provided for both mechanical reinforcement and to enable a connection to a ground. The shield is disposed flat against the casing. The braid is disposed against the shield running centrally along the shield and is stitched through to the casing using a cloth thread. In operation the shield and casing are wrapped around the conductors which are to be protected. The casing is then zipped up around the shield, and an end of the wire braid may be connected to a ground source such as a connector housing.
For shielding against low-power high-frequency emissions, a light metal foil or metal coated polymer film may be used. The use of a coated film has the advantages that the film provides strength, flexibility and a layer of insulation. The thin metal layer that may be deposited on the film does not, however, perform well in shielding against higher-power lower-frequency emissions.
It is therefore desirable that a flexible, closeable shielding jacket be constructed to offer shielding against a broad frequency range of electromagnetic emissions.